Most cathode ray tubes exhibit a center to edge light output gradient. Typically, the center light output exceeds the corner light output and is the result of a number of contributing factors. For example, the geometry or thickness of the faceplate panel glass along with the absorption characteristics of the glass. Light output gradient is also affected by other processes applied to the inside of the faceplate panel. During production of cathode-ray tubes (CRTs) for color television receivers, a black matrix is applied to the inside surface of the faceplate panel. The black matrix consists of parallel lines which extend vertically as defined by the viewing orientation of a conventional tube. Black lines are spaced at desired intervals leaving transparent glass in the spaces between the matrix lines. The transparent spaces are coated with slurries of materials containing phosphors which emit the three primary colors of light, i.e., red, green and blue, when impacted by electrons. The three phosphors are alternately applied in a repetitive sequence such as red, green and blue to all the transparent spaces of the panel. Prior to the application of the phosphors, it is desirable to measure the matrix's contribution to light output gradient so that undesirable increase in the gradient caused by the matrix can be minimized thus avoiding the expensive application of phosphors to improperly matrixed faceplate panels. It is also desirable to record the light output gradient of each panel in order to optimize the matrix application process and better match the faceplate panel with other components of the CRT.
A system for measuring transparent space width in a matrix is shown in U.S. Pat. No. 4,525,735. This system has been found to be unacceptable in predicting resultant light output gradient in a finished CRT. A problem with such a system is that the panel movement takes a relatively long time and repetitive movements and tests are required in multiple passes. This lowers the production rate to a point where it may be impossible to meet a particular desired inspection time, e.g., 12 seconds per panel. Another problem with this system is that invalid readings can be processed, which can result in the acceptance of an out-of-tolerance panel.